The use of metal salts, especially zinc salts, of phosphorodithioic acids as antioxidants and extreme agents in lubricants and functional fluids has been known for some time. However, the environment in which such lubricants and functional fluids are used has become increasingly severe over recent years with the further development of machinery employing such lubricants and functional fluids. It is important, therefore, that materials of this type be developed which have higher thermal stability than has previously been the case.
The use of sulfurized calcium alkyl phenates as compounding agents in lubricating oils to inhibit corrosion, piston ring sticking and gum formation in internal combustion engines resulting from oxidation of lubricating oil and polymerization of engine fuel residues is also known, as indicated in U.S. Pat. Nos. 2,680,096 and 3,036,971.